As a rule, the local processing of data in the local data processing device is carried out on the basis of the local timebase. This is formed in the respective local data processing device in particular by means of so-called timer modules. These are generally counters which are decremented cyclically by a local clock transmitter, emit a trigger or time-transmitter signal for the respective local timebase whenever they are completely decremented, that is to say whenever the counter contents pass through zero.
In automation engineering, local data processing devices may be used for decentralized control of distributed technical equipment which, as a component of a production facility which in some circumstances is complex, impinges, for example, on the machining of a workpiece or the processing of a raw material. By way of example, as one of many possible examples of such technical equipment, electrical drives may be mentioned, which are in each case supplied with data from an associated local data processing device; this may also be referred to as a drive controller. In the example, such data are largely control variables, that is to say, in particular, measured values, set values and settings.
For the operability of the entire production facility, for example a CNC machine tool, it is now generally necessary for the technical equipment of such a facility to act in a coordinated fashion on, for example, a workpiece or a raw material. This is in turn dependent on the local timebases in the local data processing devices of the technical equipment being synchronized to one another. This ensures that, for example, actual values are detected simultaneously by the local data processing devices and, for example, control signals are output simultaneously to the associated technical equipment. All the local data processing devices involved in a production facility thus act at the instant of a corresponding processing state on the respective workpiece or raw material in terms of metrology and control engineering.
Furthermore, regular, renewed synchronization of the local timebases in the local data processing devices is necessary. The reason for this is the associated local, in particular crystal-controlled, clock transmitters in the timer modules. These have different scatters between the units in the various local data processing devices and thus have a different long-term drift behavior. Thus, the local timebases would gradually drift apart from one another without regular synchronization.
In a first, known system, the local data processing devices can be supplied via a separate clock line with a fixed, central control clock for synchronization of the local timebases. This central control clock is supplied directly to the clock transmitter for the respective timer module. Such an arrangement is particularly complex, since the separate clock line must be routed in parallel with a data bus which is generally present in any case and connects the local data processing devices to a central data processing device.
For synchronization of local timebases in local data processing devices, they may also be connected via a data bus to a central data processing unit in which a central database is formed and maintained. In this case, in a known system of this type, so-called “clock messages” are transmitted from the central timebase to the local timebases, where they are evaluated for synchronization. However, a disadvantage that occurs in this case is that the clock messages must be supplied at strictly equidistant times from the central data processing unit to the data bus. These clock messages are received by a specific evaluation circuit in the local data processing devices, in particular by a PLL (phase locked loop) evaluation circuit. This PLL evaluation circuit derives a correction signal from the reception timing of the equidistant clock messages, and this is used to readjust the clock transmitter which acts on the respective timer module, in order to synchronize the local timebase. Synchronization using clock messages at strictly equidistant times is also referred to as hard synchronization.
A major disadvantage with such a system is that separate hardware in the form of an evaluation circuit is required in each local data processing device to receive and evaluate the clock messages which are strictly at equidistant times. A further disadvantage is that, in practice, it is often impossible to comply precisely with the requirement for strictly equidistant timing, for example owing to a particular type of data transmission on the respective data bus or, for example, owing to interrupt-dependent processing delays in the central data processing unit. This leads to fluctuations, in particular delays, in the timing of the clock messages, is also referred to as jitter. In some circumstances, this jitter propagates into the local timebases and can cause equidistant fluctuations in lower-level fine control clocks in the respective local data processing device.